Board & Card Games Stack Exchange is a question and answer site for people who like playing board games, designing board games or modifying the rules of existing board games. It's 100% free, no registration required.

4 Answers
4

They are listed among the contents (on page 2) and the main use is detailed on pg 5 of the rules.

For example, the players can shuffle the
Randomizer cards for all Kingdom cards and
draw 10 to select the cards for the game.

Or,
players can take turns selecting cards.
Players may also use the Randomizer cards
as Placeholders to mark the card piles so
empty piles are easily seen.

A lot of folks use a random deck generator, we have a list here at B&CG. If you use sleeves, it may be helpful to use a different color sleeve for the randomizer cards so that they catch your eye easier.

Do not add them into the piles unless you want to change the balance of the game. There are supposed to be 10 of each card, not 11. Some game strategies revolve around depleting piles fast or unexpectedly and you would impede those by having 11 cards in each stack. Also, since they have a different back, they will be noticeable unless you are using opaque sleeves.

They were primarily intended to be used for randomizing, but most people use some sort of application for that. However, as is suggested in the rules, they can also be used as placeholders to indicate depleted stacks. This can be useful, as it's easy to lose track of how many stacks have been used up.

They are called the randomizer cards, and they are supposed to be kept separate from all the other cards and used to select 10 random kingdom cards at the start of the game.

I personally use one of many online deck randomizer (my favorite), for this and just leave them in with the rest. You can cheat and try to bring them to the top of your deck when shuffling so you can draw them more quickly, but I've never played a serious enough game for this to be an issue. Also, leaving them in will make the game infinitesimally different since each kingdom pile has one more card. If you were running a tournament, you might want to leave them out for maximum fairness.